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My BT line gets fits of crackling and we are 2 miles away from the exchange. I have renewed the BT box 1036886 which cleared the worse and cleaned up all the pushin connectors which helped a little. The broadband seems OK but still the phone gets an occasional fit.
As I am a talktalk customer used their checker which says it is working OK but little else.
So decided to check line with voltmeter and confirmed the DC volts from true earth are A=+3.7 volts and B=+55.5 to 50.3 but will not stabilize. But the DC volts between A and B is 51.9 most of the time but occasionally it drops to 49.8 or surges up to 53.6 causing crackling. The AC range of digital voltmeter reads mostly about 0.7 volts varying about 0.1 volts occasionally between A ad B.
If I install a sacrificial anode by earthing my A wire will this help??

Do you get crackling on the line if you plug a phone directly into the master socket [behind the faceplate on the NTE5] and run a quiet line test [dial 17070 and select option 2 if I recall correctly].

If you do then sounds like a contact fault somewhere on the line and would require BT to identify and fix [BT still maintain the physical wires via the Openreach brand even if your contract is with talktalk for line rental although it would have to be arranged through talktalk]. It also sounds like you've done works on the master socket which isn't allowed - everything from the master socket back to the exchange belongs to Openreach so installing a sacrificial anode on the exchange side of master socket is likely to land you with a large bill if it gets discovered.

Thanks Jake
Have tried the option 2 quiet line test and crackling is still there. The problem could be a dirty contact somewhere on the BT pair along the cable route OR it could be a spare wire sometimes touching or upsetting the magnoflux field that surrounds my A-B pair which forms a sort of DC tunnel in which the frequency vibration is transmitted.
Funny that it does not upset the internet more as I seem to remember that broadband is 20kH to 200kH being above the 100H to 15kH used for voice frequencies. I wonder why the internet is stable?

I had the Telcom chaps out and they have given me another pair. DC volts now A=-55.6 B=-5.6 volts and steady as a rock.
BTW if I lift the handset the volts change to A= -33 and B=-22 volts.
No more crackling but still cant understand why the internet was not affected.
CliveS

Thanks BillE,
I seem to remember in the 80's that computers kept getting "file error" which meant we had to start again. Nowadays it seems that the HTTP automatically gets the last corrupted packet to be resent. Very clever. Also I believe that our exchange has just been unbundled; whatever that means! Anyway it has certainly speeded up the internet connection through Talktalk.
CliveS